Simmy's Naked Turbo Project

I finished another fluid measurement with the new head gasket.
This time I measured out 32 cc's as accurately as I could.
This filled the chamber exactly to the bottom of the spark plug threads.
After settling and tapping the head I fit 1 more cc in.
33 cc's it is.
As it stands now the CR is 6.8:1

By shaving the block;
1.7 mm makes 8.2:1 - same as stock Seca Turbo
1.6 mm makes 8.1:1
1.5 mm makes 8.0:1
1.4 mm makes 7.9:1
1.3 mm makes 7.8:1 - same as Kaw Turbo

Not sure which one to pick?
Much bigger non-forged pistons,
I'm leaning towards the lower number.

Not sure if I'm going to worry about that sharp edge in the head.

 

Kawasaki Turbo 7.8:1 CR 10.5 psi boost
Honda CX500T 7.2:1 CR 19.3 psi boost
Honda CX650T 7.8:1 CR 16.4 psi boost
Suzuki XN85 7.4:1 CR 9.6 psi boost

My Seca currently measures 9 psi and stock CR is 8.2:1

 

The cr is only part of what could prove fatal - detonation/knock is what will send your combustion temps skyrocketing, and melt a piston in about 5 seconds (I have that tshirt). Weak mix is one factor, too much ign timing advance is another.
Too low a cr will make it sluggish at the bottom end though...
8.2 should be fine, provided the tci retards the timing according to boost - does it, and by how much?
I wouldn't worry about the step at the head either, might give you some extra turbulence which will help.

 

You mentioned the tdi(tci right?) curving the ignition timing as it builds boost? Are non turbo ignition adjustable?
@Simmy the sharp edge your talking about is the diameter of the 650 combustion chamber in thr 900 bore right? That was a concern I had too. 900 heads have fewer options in terms of cams and intake. They also take up more space right?

 

Nothing yamaha made is adjustable. I don't even know for sure the turbo tci retards on boost, but I would expect it to. My vmax adjust according to manifold vacuum, or it would if the little sensor inside wasn't rattling around..

 

This shows everything feeding into the Turbo's ignitor unit.



this is a real complicated flow chart LOL.

 

I literally spent 2 minutes with a rotary file then took another fluid measurement.
The change in volume was hardly discernable.
I'm still undecided if I'll do this to the head, maybe just round it with sand paper.

 

Don’t is my advice, just take the sharp edge off - most if not all car cylinder heads have a square edge, if I skim a head I will carefully run a needle file around it, job done, and these would be race engines.
Re the ignition tci, it must have a vacuum-pressure variable adjustment built in as well as antiknock, quite advanced for its day.

 

Hey @Simmy I just now ordered a cylinder. It's the 68.5 go big or go home. I'd like to run a murray carb set up but he doesn't make one for the 900 I gather the spacing Is different. I'm going to use the yics port on the 900 so I need a head because I've got a 1980 pre yics. Does a 750 head have bigger valves or anything that would make it the better option opposed to the 650

 

That's what I will do.

You could use your 650 non-YICS head. The YICS passageways in the block will just lead no where.
The 650 and 750 use the same valves.
You might want to compare the head volumes as you will need to raise compression more for a normally aspirated motor.
I suspect your 650 head might be tighter in this regard and the better choice.

Why would you go to the trouble of the big bore then take a step back performance-wise with the Murray carbs?
I would think a 772 should work well with the stock 750 jetting.

 

Is it really a step backwards? If you can send some good links on the subject I'd read em I I figured the constant pull of air would be beneficial

 

Was also thinking doing something along the lines of putting two filter in the air box one for 1 and 2 and the other running 3 and 4.
I'm aware that it's not a small block and more air in isn't always better

 

I don't have factual information, just assumed 4 individual carbs would be better.
What does Murray claim?

 

Of course it is, at anything approaching full throttle and beyond. For pooling about town, no issue, but out on the open road it sure will be. Maybe a bigger engine will compensate

 

Look up xj650 murray carb it'll take you straight to his site.
Gentlemen I don't mean to be niave. My thought behind it is that during the 3 strokes that do not create vacuum. air traveling threw a single carb per cylinder setup would lose momentum or cease completely. If the second cylinder firing opposite to it is drawing air threw the same carb then less lose takes place. That's just what I've come to know. But I'm completely new to these bikes and I'm up for hearing your thoughts and experiences. And if you hit me with sources I like to read that kinda suff.
Similar to the scavenging affect of good header compared to factory cast manifolds. I've read a few thing on cv vs cr carbs still have quite grasped it yet tho

 

Explain why it might not be good on open road?
2x36mm carbs means over all less volume then 4x28 or whatever the hitachis are so it makes sense that it responds better at lower rpm. But would you really lose top?

 

With single carb per cylinder you can use pulse length tuning, since there are no conflicting waves. You have no bends, tees etc, so less losses. Yamaha were experts in sound and pulse tuning.
On the negative sides of twin carbs your intake pulses are not even, unless you have acrossover manifold? I must confess never to have looked the manifold up. But if it worked anything like as well don't you think Yamaha would have offered it - two carbs less etc?

 

Small change in plans.
I talked to my engine guy.
He said if he removes the sleeves and puts them back he can't guarantee they will remain round.
I had worried about this but he said he would match mark them so they go back in exactly the same position.
He wondered if the piston clearance would allow a good honing to true them up afterwards.
Once I got home I got thinking, hmmm...
I also have an 853 cylinder block, why not use it and bore it from 67 to 68.5.
I showed him the 853 block, he measured the sleeves and determined it would be no problem.
The sleeves will still be about .250" thick after boring.
He has seen cylinder sleeves bored to .090" thick without problem.
However they weren't likely turbocharged.

Not going to be cheap but this is certainly the way to go.
A nice fresh bore will be great, perhaps new rings to.
No rush, still lots of work to finish the bottom end, mostly just new gaskets, inspect the clutch.
I'm not planning to split the cases at this time.
Fingers crossed that the starter clutch and primary chain guide are good.
New cam chain with master link.
And restoring a head.
The plan will be to have the 772 running next spring.

 

Wow! Living on the edge........

 

For as far I as you are and engine is out of bike, I think I’d just split and make sure. Like you said not in a rush and I’d kick myself for sure if I skipped this and had a problem once in bike.